Color synchronizing signal separation



Oct. 1,1, 1960 R. w. soNNENFELDT 2,956,111 y COLOR SYNCHRONIZING SIGNALSEPARATIONV Filed April 29, 1952s l 2 sheets-sheet '1 B+ l5 j F j H0750?k @lain/riz 4 mmv .fm/I4 .sal/lef mi 50 aww/7' mw( INVENTOR.

Mr oRNEYl Oct. 1l, 1960 R.w.vsoNNENFELDT 2,956,111 l COLOR SYNCHRONIZINGSIGNAL SEPARATION Filed April 29,'1953 2-Sheets-Sheet 2 INI/ENTOR.

d TTORNE Y United States Patelff Q13 ice.v

lCOLOR SYNCHRONIZING SIGNAL SEPARATION Richard W. Sonnenfeldt,Haddonfield, NJ., assignor to Radio Corporation of America, acorporation of Delaware Filed April 29, 1953, Ser. No. 351,776

6 Claims. (Cl. 178-5.4)

This invention relates to apparatus for the separation of signals `froma composite wave and more particularly to the removal of the colorsynchronizing burst in color television systems. Y

In the color television system as developed by the National TelevisionStandards Committee a burst of a reference frequency, which is thefrequency of the color subcarrier, is positioned on the back porch ofthe horizontal blanking signal. The back porch of the horizontalblanking signal is that portion of the flat top of the blanking pulsebetween the trailing edge of the horizontal synchronizing pulse and itsown trailing edge. Each cycle of the burst swings symmetrically aboveand below the back porch level according to the specification adopted byN.T.S.C. Panel 14 on May 20, 1952. To 4insure proper synchronization ofthe demodulating oscillator in the color receiver the phase of thetransmitted burst is compared with the phase of the locally generatedreference frequency. Should there be a difference, a phase detectingapparatus derives an error voltage which may be used, for example, torestore the local demodulating oscillator to the proper phase andfrequency relation. Failure to achieve exact synchronization results inunpleasant images on the viewing screen because of improper colorfidelity.

Early methods of separating burst (which was first positioned on apedestal so that the tips of the negative half cycles just touched theback porch level) involved a procedure wherein the horizontalsynchronizing signal and burst were clipped from the composite colorvideo signal and then the burst was gated out by means of a delayedpulse derived from the horizontal synchronizing signal itself. Thissystem had the attendant difliculty of permitting noise to gate thecomposite video signal at improper times. Random noise pulses might besuficiently large so that the synchronizing signal separator would becharged up and would be operative during the interval of the noiserather than during the synchronizing interval. Not only did the gatingoccur at an improper time but in addition the gating was renderedinoperative during the occurrence of the synchronizing signal.

Accordingly other means of effecting burst separation were sought andone useful method was the employment of a portion of the horizontaldeflection signal as the source of the gating signals. Since thehorizontal deflection system of a television receiver is generallycontrolled by one of a variety of automatic frequency control systems,there is less susceptibility to noise interference. The synchronizingpulses are integrated and in this way the actual average deflectionfrequency is approximately the correct standard frequency.

. However, there were other difficulties which arose when the horizontaldeiiection signal was used as the gate pulse source. The phase relationof the horizontal defiection signal to the incoming synchronizing signalis a function of the tuning of the horizontal deflection oscillator.Changes in the hold control setting of the horizontal deflectionloscillator therefore affect. this phase relation-V ship and it istherefore advantageous to provide a gating signal which is wide enoughto encompass all' possible phase differences within the range ofvariation of the ,hold control. Gating signal widths of the order ofoneand-one-half or two times as great as the burst duration having beenemployed which are at the same time suciently narrow to preclude thepossibility of gating out picture information as well.

While it is true that video information is not introduced, the extraVwidth makes for greater possibility of noise interference. `A possibleway of overcoming this disadvantage is by improving the stability of thehorizontal deflection oscillator so that the effect of variations inhold settings is minimized permitting the use of a narrower gatingsignal, However these refinements may entail a considerable expenditureof money, may involve rather relaborate circuitry and require the use ofprecision components. Y

The present invention includes apparatus for deriving from thehorizontal deflection signal an essentially rectangular gating pulsehaving a Width somewhat greater than the duration of the burst. 'Ihegating pulse is added to the burst which swings symmetrically above andbelow the top of the pulse andthe resulting wave is applied to a peakamplifier. In the peak amplifier only a portionv of theburst appearingatop the gating pulse is amplified and applied to a circut tuned to theburst frequency where the complete burst waveform is restored. In thisway even, though the gating pulse width is wider than the burst, thepeaking amplifier removes and affects only a part of the burstpositioned atop the gating pulse so that noise occurring during theextra width of the gating pulse cannot enter the system.

It is therefore an object of the present invention to provide animproved means for separating a significant signal of a certainfrequency from a composite wave in which the significant signalperiodically recurs. Y

It isa further object of this invention to provide a relai tively noisefree means for separating the burst component from a composite colorvideo Wave containing the burst at a recurring rate.

These and other objects of the invention will become apparent from adetailed consideration of the drawings inwhich: Y

Figure 1 is a circuit diagram of one form of the Vpresent invention;

Figure 2 is a series of curves representing waveforms at various pointsin the circuit of Figure 1; Y Figure 3 is a block diagram of anotherembodiment of the present invention; and ,l A

Figure 4 is the circuit diagram corresponding to the block diagram ofFigure 3. v

Figure 1 shows one form of the invention. vFrom a horizontal deflectionsignal source 1, which may be the deflection yoke of the kinescope,horizontal fiyback voltage pulses are impressed via condenser 4 upon thegrid 2- of tube 3. The iiyback voltage pulses may alsobe derived fromthe horizontal output transformer or the hori-V zontal output amplifiertube of a conventional television' receiver. The horizontal iiybacl:Ypulses of voltage aregenerated in the defiection circuits of thetelevision receiver during the retrace interval of the scanning. Dur-Vthe. current is reversed in a very short time and its rate of change isnot constant and a pulse of voltage will be generated in the deflectioncircuits. This pulse of voltage.Y

is called the fiyback pulse.v Condenser 4 and resistor 5 differentiateytheinput flyback pulses shown in curve B1 of Figure 2 to produce thespiked waveform pictured in Planted oet.A A11, 19Go curve B2 of Figure2. It will be noted that the curve B1 has a shape somewhat like a sinewave. If this wave is differentiated, the mathematical derivative willbe a cosine wave. A cosine wave is substantially the form shown by curveB2. Tube 3 clips the shaded portions of curve B2 so that at plate 7 oftube 3 the waveform is as shown in curve C of Figure 2. It is to benoted that the trailing edge of the negative pulses 6 as shown in curveC of Figure 2 (or the leading edge of the positive pulses 9) occurs at atime shortly before the occurrence of the trailing edge of thesynchronizing pulse 8 shown in curve A of Figure 2. The positive pulses9 shown in curve C of Figure 2 likewise have a width which extendsbeyond the last cycle of the burst. The positive pulses 9 arehereinafter referred to as gating pulses.

Instead of applying the positive pulses 9 to a grid of adder tube 10they are applied directly to plate 11 through damped tuned circuit 33which is resonant to the burst frequency and greatly improves gain. Thistuned circuit is highly desirable but not essential to the operation ofthis form of the invention. The composite color video signal which isshown in curve A of Figure 2 is applied to control grid 12 of tube 10 byway of coupling condenser 13. In one form of the invention tube 10 mayhave its plate 11 operated at a voltage which is less positive than thevoltages on its screen grid 14 so that until a rectangular pulse isapplied to plate 11 no plate current is drawn. In this fashion duringthe interval between gating pulses the feed-through is insignificant andlittle noise can enter the system. Although this requires more inputpower, the noise eliminating qualities may be as much as two or threetimes superior to conventional methods. A 6AH6 or similar tube issuggested for use as adder tube 10.

Although the above method has good noise rejection characteristics, itis not essential that tube 10 be cut olf between the occurrence of therectangular gating pulses. By proper adjustment of plate resistor or byreducing the potential applied to screen grid 14 the plate voltage oftube 10 may be made somewhat higher than the screen grid potential sothat the rectangular gating pulses may be simply added to the compositevideo signal from source 19 during the occurrence of burst, thus raisingburst far above the tips of the horizontal synchronizing pulses.

At the plate 11 of tube 10 the waveform shown as curve D of Figure 2which may be termed pedestalled burst signal, resulting from theaddition of curves A and C of Figure 2 will appear. It is to be notedthat the axis of the burst lies along the top of the gating pulse andonly the positive half cycles are situated above the pulse top. Thepedestalled burst signal is applied through condenser 16 to the controlgrid of tube 18 which is gridbiased and which is operated so that onlythe peaks of the applied signal are amplified and passed. Noise whichmay be present on top of the gating pulse on both sides of the burstwill thus be largely ignored by the peak amplifier 18. The positivehalf-cycles of current from the tube 18 are applied to tank circuit 20comprising inductance 21 and condenser 22. It is to be understood thatthe tank circuit can operate even though the current fed to it is lessthan full half cycles. Therefore so long as the peak amplifier operatesabove the A.C. axis of the burst atop the gating pulse the tank willrestore the complete sine wave. Resistor 23 shunts the tank 20 andserves as a damper. The tank 20, being tuned to the burst frequency,restores the full sine wave which is fed to the output. Tube 18 may be a6AH6 or similar tube. The time constant of condenser 16 and resistor 17is made long compared to the interval between flyback pulses and thegating pulses derived therefrom. Grid bias on tube 18 is preferred tofixed bias so as to account for varying levels of input. In practice,when condenser 16 had a capacity of .001 microfarad and resistor 17 hada megohm of resistance the desired results were obtained.

The tank circuit 20 worked very satisfactorily when it was designed tohave a 600 kc. bandwidth between its half-power points, which isequivalent to a Q of l0 or 16.

Another form of the invention is pictured in Figure 3. The source 1 ofhorizontal deflection signals feeds iiyback pulses to a differentiator25 whose output wave is as shown. Clipper 26 removes the shaded portionsand amplifies the rest so that negative and positive substantiallyrectangular gating pulses appear in its output as shown. From asynchronizing signal separator 24 of the receiver the horizontalsynchronizing signals plus the envelope of the burst are derived asshown and applied to the first of three inputs of adder 27. The negativeand positive rectangular gating pulses from clipper 26 are applied tothe second input and the composite video color signal from source 19 isapplied to the third input. The resultant combined wave takes the formshown at the output of adder 27 and it is seen that the burst is nowpositioned on a pedestal 32 formed by the detected envelope which itselfis located atop the positive gating pulse derived from clipper 26. It isalso seen that the width of the pedestal which is itself derived fromthe burst is substantially identical with that of the width of the burstso that there is no extra width of the pedestal top on either side ofthe burst on which extraneous noise signals could be located. Peakamplifier 28 then passes and arnplifes only the positive half-cycles ofcurrent as shown (or any portion of the burst lying above the A.C. axisor the burst as explained above) and the latter are then applied to tankcircuit 20. This is tuned to the burst frequency so that the completesine wave cycles of the burst are restored and applied to the outputwhich may be, for example, one input to a phase comparison device forcontrol of the local demodulating oscillator.

Figure 4 shows one circuit which embodies a form of the inventionexplained in connection with Figure 3. Flyback pulses from horizontaldeflection signal source 1 are differentiated by the grid leak biascircuit of adder tube 29 which clips the differentiated wave to producesubstantially rectangular gating pulses. Composite video signal source19 feeds the composite color video wave to the plate of adder tube 29.synchronizing signal separator 24, which may be any conventional type,applies the horizontal synchronizing signal and the envelope of theburst to the plate of adder tube 29. This plate mixing arrangement ishelpful in preventing feed-through of unwanted signals as explained inconnection with Figure l. The composite wave including the resultingsignal shown in Figure 3 as the output of adder 27 is impressed viacondenser 30 to the control grid of tube 31 whose operatingcharacteristics are such that only peaks of the input wave are passedand amplified. As a result only a portion of the burst appearing atopthe positive rectangular pedestal caused by the addition of the detectedburst envelope pedestal 32 to the gating pulses shown in the output ofclipper 26 in Figure 3 is amplified. The ampliied half-cycles or less ofcurrent at the plate of peak amplifier 31 cause the tank circuit 20,which is tuned to the burst frequency, to oscillate at the burstfrequency. Resistor 23 is used for damping purposes. The output is takenacross the tank circuit 20 and fed to an appropriate phase comparisondevice.

In this latter form of the invention it is thus seen that an addedrefinement is introduced. Even if there is a phase shift in thehorizontal deection signal source 1, which may ,bel due to a change inthe hold setting resulting in a corresponding shift of the rectangulargating pulse derived therefrom, the pedestal 32 derived from theenvelope of the burst and occurring at substantially the same time asthe burst will merely be shifted in time on top of the rectangulargating pulse. Since the burst is then placed on the detected burstpedestal 32 the phase shift has no undesired effect. Furthermore sincethe pedestal. 32 has a time base shorter than the width of the gatingpulse and is situated above the top of the gating pulse, noise on thegating pulse will be largely ignored and will not be passed through thepeak amplifier tube 31. This added noise immunity is obtained byv simplyusing a triode and a pentode as shown since all the other requisitesignals are already present in a television receiver. It is thus anextremely economical method for producing noise-free burst separation.

Having thus described the invention, what is claimed is:l

1. Burst separation apparatus for a television receiver comprising incombination a source of horizontal voltage iiyback pulses, means coupled.to said source for differentiating said iiyback pulses, means coupledto said differentiating means for clipping said differentiated flybackpulses to produce essentially rectangular pulses, a source of compositecolor video waves in which a burst of a subcarrier frequency signalrecurs periodically on l the back porch, means for detecting theenvelope of said bursts, means coupled to said clipping means, to saidcomposite color video wave source and to said envelope detecting meansfor yadding said rectangular pulses, said envelopes and said Ibursts toform a comhined wave in which por-tions of said bursts are positionedatop said envelopes, said envelopes being positioned on said rectangularpulses, means coupled to said adding means for amplifying substantiallyonly said burst portions appearing atop said envelopes, and frequencyresponsive means coupled to said amplifying means for restoring saidbursts in response to said amplified burst portions.

2. In a color television receiver adapted to receive a color ltelevisionsignal having a blanking interval which includes a horizontalsynchronizing pulse having a first polarity which occurs during a firstportion of each blanking interval and a color synchronizing burst havinga burst frequency and phase which occurs during a second and laterportion of each blanking interval, the combination of: means responsiveto said horizontal synchronizing pulse to develop a pair of pulsesduring each blanking interval comprising a first pulse occurring duringsaid first portion of said -blanking interval and having a secondpolarity followed by a second pulse occurring during said second portionof said blanking interval and having said first polarity; means to addsaid pair of pulses to said color television signal to produce apedestalled burst signal wherein said bursts are pedestalled on saidsecond pulse and wherein said first pulse and said horizontalsynchronizing pulse forms a waveform having a peak amplitudesubstantially less than the peak amplitude of said pedestalled burstsignal; amplifier means coupled to said las-t named means and -biased to=be responsive to only peaks of first polarity of said pedest-alledburst signal yto develop a reconstituted pedestalled burst from saidburst signal peaks.

3. In a color television receiver adapted to receive a color ltelevisionsignal having a blanking interval which includes a horizontalsynchronizing pulse of a first polarity occurring dur-ing a firstportion of each blanking interval and a color synchronizing burst havinga burst frequency and phase occurring during -a second and later portionof each blanking interval, the combination of: means responsive to saidhorizontal synchronizing pulse to ldevelop a pair of pulses during eachblanking interval comprising a first pulse occurring during said firstportion of said blanking interval and having a second polarity followedby a second pulse occurring during said second portion of said blankinginterval and having said first polarity; means to add said pair ofpulses to said color television signal to provide a pedestalled signalwherein said bursts are positioned on said second pulse; amplifier meanscoupled to said last named means and responsive to said pedestalledburst signal to conduct alternating current wave information during saidsecond portion of said retrace interval to thereby develop areconstituted burst from said pedestalled burst.

4. In a color television receiver adapted to receive a color televisionsignal h-aving a blanking interval which first polarity and which occursduring a yfirst portion of each blanking interval, and which includes acolor synchronizing burst having a burst Afrequency and phase and whichoccurs 'during a second and later portion of each .blanking interval,the combination of: first means. responsive to said horizontalsynchronizing pulse to develop a pair of pulses during each blankinginterval, said pair of pulses comprisinga first pulse having said secondpolarity and occurring duringsaid first portion of said blankinginterval followed by ia second pulse having said first polarity andoccurring during said second portion of said blanking interval; secondmeans responsive to said color synchronizing Iburst in said colortelevision signal to detect said burst and to derive from saiddetect-ion -a pulse of second polarity which occurs during the timeinterval that each burst occurs, a wthird means to provide said colo'rtelevision signal; signal combining means coupled to said first, secondand third means and .responsive to said first, seco-nd and third pulsesand to said color television signal to add said rst, second and thirdpulses Ito said color .television signal to produce a pedestalled burstsignal wherein said bursts are pedestalled on said second and thirdpulses and wherein a combination of said first pulse and said horizontalsynchronizing pulse forms a waveform having a peak amplitudesubstantially less than the peak amplitude of said pedestalled pulse;amplifier means coupled to said last named means and including biasingmeans whereby said amplifier means is responsive Ito only .the peaks ofsaid pedestalled burst signal to develop oscillations having a frequencyand phase related to said `burst lfrequency and phase from said burstpeaks.

5. In a color television system in which lthere is a color video wavecontaining periodically recurring bursts of a subcarrier frequencysignal and a source of horizontal tiyback signals, the combination of:means to differentiate said horizontal fiyback signals to develop afirst waveform of first polarity which occurs during a first portion ofeach scanning retrace and to develop a second waveform having secondpolarity which occurs during the second and later port-ion of eachscanning retrace interval, clipping means coupled to saiddifferentiating means for deriving first and second substantiallyrectangular pulses having respectively first and second polarities fromsaid first and second waveforms respectively, signal adding meanscoupled to said clipping means and responsive .to said first and secondpulses and to said color video wave .for adding said first and secondpulses to said color video wave to provide an output wave wherein aportion of said burst is positioned atop said second pulse with a peakamplitude which is substantially larger .than the voltage amplitude atany other point in said color video wave, amplifier means coupled tosaid signal adding means and responsive only Ito the peaks of said burstpositioned atop said second pulse to develop oscillations having alfrequency and phase prescribed :by said bursts.

6. In a color television receiver adapted to receive a color televisionsignal having a blanking interval including a horizontal scanning pulseof a first polarity and peak amplitude occurring during a first positionof each blanking interval and yfurther including a color synchronizingburst signal comprising a series of oscillations of a .given frequencyand peak amplitude less than the .amplitude of said horizontalsynchronizing pulses occurring during a second and later portion of eachblanking interval, the combination of: means responsive to saidhorizontal synchronizing pulses for deriving a gating pulse, means forcombining said ygating pulse and said color synchronizing burst toprovide a pedestalled burst sign-al having a peak amplitudesubstantially larger than the peak amplitude of said horizontalsynchronizing pulse and of the same polarity, and means responsive tothe 7 peaks `of's'ad pedestalle'dburst signal for'deving os1' OTHERREFERENCES 1 1911011591 Sald burst frequency' Radar El'eotranicFundamentals Navships 900,016, June 1944, page 52. (Copy in Patent Ofce,Div. 48.)` References Cited m the me of this patent waveforms-MJT.Radiation Laboratory Series, vol'. 4 UNITED STATES PATENTS 5 19, 1949,Ipage` 9. (Copy in U.S. Patent Oce Library.) r2,490,025 Bryan Dec. 6,1949 Radio. Engineering vby- Termap, 1947, pages 601-602. 2,594,380 Bammet a1. Apr, 29, 1952 (Copy an US Patent Ofic@ L1brary) 2,751,430 Kellylune 19, 1956 2,873,311 Sormenfeldt Feb. 10, 1959 10 2,879,328 Larky 1Mar. 29, 1959

